The present invention relates to a circuit arrangement for evaluating received oscillations of a first predetermined frequency by determining the time duration of the oscillations, which duration constitutes information, with the information itself being evaluated in a subsequent control circuit oscillations of a different frequency which are received subsequent to the oscillations of the first predetermined frequency. More particularly the present invention relates to such a system wherein the received oscillations are ultrasonic oscillations used for the remote control of a television receiver.
It is known that information, e.g. information for the operation of an instrument, can be transmitted over wires or without wires. It is possible, for example, to operate a television receiver by means of a wireless ultrasonic remote control system. Special measures are then required to prevent the receiver from responding to inadvertent erroneous instructions which could be produced by interfering frequencies not emitted by the ultrasonic generator of the control system.
For this reason the ultrasonic remote control device disclosed in the periodical "Funkschau" 1973, Issue No. 18, pages 675-677, utilizes an ultrasonic signal which contains the desired operating function and which consists of two immediately successive frequencies. The first frequency F.sub.1 or F.sub.2 of the ultrasonic signal determines whether a channel is being selected or a level is being changed, while the time duration of the ultrasonic signal at the first frequency determines the number of the channel being selected or the type and direction of the change in level, respectively. The performance of the previously recognized and stored instruction is not actuated, however until the second immediately following ultrasonic signal at frequency F.sub.3 is received. If in the known ultrasonic remote control system an interference signal occurs at frequency F.sub.1 or F.sub.2, this signal can be received by the ultrasonic receiver but the instruction is not performed unless frequency F.sub.3 appears immediately thereafter. In this way the production of erroneous functions in the television receiver caused by interfering frequencies is avoided.
In this known ultrasonic remote control system a multivibrator is operated during the presence of frequency F.sub.1 and two decimal counters count the pulses from this multivibrator. If the first received frequency had the value F.sub.1 then the number of counted pulses constitutes the channel number. If, however, the received frequency had the other value F.sub.2, the number of the counted pulses determines the direction and type of change of level. The outputs of the two decimal counters are connected to a control circuit to effect the change in level or channel selection, respectively. When subsequent to the first frequency F.sub.1 or F.sub.2 the other frequency F.sub.3 is received, the counters are unlocked. The stored counter contents now switches one of its outputs so that the control circuit can perform the instruction associated with that particular output.